A wide variety of small motion detectors have been developed for the express purpose of sensing acoustic signals in the ocean. Generally speaking, these hydrophones have used ferroelectric, piezoelectric, etc., materials which provide representative output signals when acoustic pressure waves induce strains in them. These representative signals are fed over wires to a signal processing system. Unfortunately, the transducers usually are delicate, expensive and susceptible to electromagnetic interference.
A new family of hydrophones has been developed which rely on optical fibers. Many of these operate by sensing a change in the refractive index of the fiber as the acoustical pressure changes. The change in refractive index caused by the changes in pressure modulates the phase delay of a light beam passing through it which later is detected by means of an optical interferometer. Such is the mode of operation relied upon in U.S. Pat. Nos. 3,920,982 and 3,625,589. Although most of the problems associated with long electrical conductors and power sources are avoided and both of these devices appear to be promising, the degree of sensitivity provided may be questionable.
Thus, there is a continuing need for a hydrophone preferably relying upon fiber optic technology to provide a sensitive monitoring instrument adaptable to be operated at great depths.